In casting technology, casting molds and cores of any considerable size are made from material mixtures containing a grainy base (in most cases sand) mixed with a polyurethane-based, cold hardening binder system. Such a binder system is composed of polyisocyanates with at least two NCO groups in the molecule and polyols with at least two OH groups in the molecule as reaction partners, as well as a tertiary amine or in some cases a chelate compound as an accelerator. Depending on whether the so-called "cold hardening process" or the so-called "gas hardening process" is being used, the accelerator will be added to the mixture either (a) together with the other ingredients of the binder system immediately before the mixture is to be used, or (b) only after the mixture, made up without the accelerator, has been put into a casting box and the mixture in the box is then treated with a gaseous tertiary amine accelerator.
A binder system of this sort generally also includes a solvent, especially when one or both reaction partners are present in a higher molecular form such as prepolymers. Thus, for example, resins of condensation made of phenols or phenol-related compounds with aldehydes are quite well-suited polyols, which regularly require a solvent on account of their relatively high molecular weight.
Although the solvent does not participate in the reaction between the polyisocyanates and the polyols to form urethanes, it nevertheless exerts an influence on the course of the reaction, which is probably related to the fact, among others, that the two reaction partners have varying degrees of compatibility with the various types of solvents. In general, polar solvents are well suited for phenol resins and similar polyols but less compatible with polyisocyanates while the opposite is true of non-polar solvents. In practice, therefore, mixtures of polar and non-polar solvents are normally used, the proportions being adjusted to suit the particular binder system used. The individual ingredients of this mixture should not have too low a boiling point so that the solvent does not lose its effectiveness too soon through evaporation.
For non-polar solvents, aromatic hydrocarbons which are usually in the form of mixtures with a boiling point above ca. 150.degree. C. (at normal pressure) are preferred; and for polar solvents, aliphatic and cyclic ketones, fatty acid esters, acetals or ketals, glycol esters, glycol ether esters, glycol diethers and similar types of compounds having a sufficiently high boiling point have been used.
However, all of the above-named polar solvents have a serious disadvantage, in contrast to the non-polar solvents mentioned: they have an extremely unpleasant smell and thus make for unpleasant working conditions, which cannot generally be remedied by special hoods or the like. In this connection it should be pointed out that resins have been developed that have only a slight unpleasant odor (which might, for example, be due to a residue of free formaldehyde), so that the solvents are in fact the principal source of unpleasant smells on the job. Solvents with no odor and with otherwise satisfactory characteristics are, therefore, urgently necessary.
A first step in this direction has been disclosed in Austrian Pat. No. 342,794 in suggesting the use of phthalic acid dialkylesters (preferably o-phthalic acid), which are liquid at room temperature and have an alkyl radical of from one to twelve and typically from three to six carbon atoms. Such phthalic acid esters are quite odorless, if not completely so. They have the additional advantage that they are more compatible with polyisocyanates than, for example, isophoron (a cyclic ketone frequently used as a solvent) and therefore lead to casting forms with somewhat better characteristics. Of course, their compatibility with polyisocyanates is still not optimal, and they have the additional disadvantage that they crack easily during the casting process, which leads to sublimation with a lot of smoke and correspondingly strong smell.